*The problem is that the manufacturers in the US are not ‘thinking metric’. If the cereal package was a logical metric size (say 500 g), then the issue starts to look much easier. The unit price then would (most likely) be in $/kg – in your example $7.72/kg.*
That's a good point--one that I had overlooked. Another that you mentioned (or at least implied) was that rounding to the nearest 100-gram point could also be helpful. I do round up or down with tenths of an oz, but the ounce numbers themselves are so small that it only makes sense to do so when rounding 19 or 21, to 20. Other than that, there's really no need to. But rounding the gram numbers would be a good start. I have a sense that rounding 21 to 20 isn't going to change the per-ounce calculations too much, but I had no feel for how big of a change moving 368 to 400 would be. Just for the record, if I can live with rounding 21 to 20, I can live with rounding grams up or down roughly 30 (Google tells me that 21 oz is 595 grams, whereas 20 oz is 567 grams. As I said, I was hesitant to do that because I had no idea of how big of a scale I was operating. Now I know.
